In conventional image forming apparatuses using an electrophotographic system, such as printers and copying machines, image density fluctuates as a result of fluctuation of environment such as temperature and humidity or a change with an elapse of time. In order to prevent the fluctuation, after a plurality of test patterns of a high/low density region are formed, respective image densities (amounts of an adhered toner) are detected, and a difference from a predetermined target value of each pattern is calculated. A feedback control for repeatedly controlling voltages such as charging voltage and developing voltage is performed until a predetermined image target density is obtained.
However, in the voltage control for correcting the voltage using two points in the high and low density regions as target values, there occurs a problem that a density fluctuation of an intermediate tone cannot exactly be corrected. Particularly in a color copying machine or a color printer, the density fluctuation of the intermediate tone remarkably appears as a change of tone.
An apparatus for performing a potential control or an exposure control of the surface of a photosensitive drum in order to obtain stability of the intermediate density is already known. However, a potential sensor for detecting the potential of the surface of the photosensitive drum of a developing time and a sensor for detecting the amounts of the adhered toner after developing are both mounted in the apparatus. These potential sensors are relatively expensive, and increase the cost of the apparatus.
An object of the present invention is to provide an image forming apparatus which corrects a density fluctuation of an intermediate tone by an environment fluctuation or a change with an elapse of time without using an expensive potential sensor.
In the present invention, a potential of the surface of a photosensitive drum is estimated based on information of various sensors, and thereby image density including an intermediate density is stabilized without mounting a relatively expensive potential sensor.
According to one aspect of the present invention, there is provided an image forming apparatus comprising: an exposure unit which generates a laser beam corresponding to an image signal; a photosensitive member which is irradiated with the beam generated by the exposure unit and thereby forms an electrostatic latent image; an electric charger to which a charging grid bias voltage is applied and which charges the surface of the photosensitive member with a uniform potential; a developing unit to which a developing bias voltage is applied and which attaches charged toner to the electrostatic latent image formed on the photo-sensitive member to develop the image; an estimating section which estimates photosensitive member properties including a potential property of an exposed portion of the surface of the photosensitive member irradiated with the light beam and a property of an unexposed portion; a contrast potential setting section which sets a contrast potential indicating a difference between the developing bias voltage and an exposed portion potential; an image forming condition setting section which calculates the developing bias voltage and the charging grid bias voltage for achieving the contrast potential based on the photosensitive member property and thereby sets an image forming condition; a high-density pattern forming section which forms a high-density pattern on the image forming condition; a judging section which compares a density of the high-density pattern with a high-density target value and judges agreement of both values; a contrast potential changing section which changes the contrast potential in case of disagreement of both the values; a re-setting section which calculates the developing bias voltage and the charging grid bias voltage for achieving the changed contrast potential based on the photosensitive member property and thereby sets the image forming condition again; and a determining section which repeats operations of the high-density pattern forming section, the judging section, the contrast potential changing section, and the re-setting section to obtain the agreement of the density of the high-density pattern with the target value, and determines the contrast potential, the developing bias voltage, and the charging grid bias voltage for a high density.
According to another aspect of the present invention, there are further provided: an intermediate-density pattern forming section which maintains the determined contrast potential and forms an intermediate-density pattern with a predetermined laser power; a judging section which compares the density of the intermediate-density pattern with an intermediate-density target value and judges the agreement of both the values; a laser power changing section which changes the laser power in case of disagreement of both the values; a re-estimating section which re-estimates the photosensitive member property in response to a change of the laser power; an image forming condition changing section which calculates the developing bias voltage and the charging grid bias voltage for achieving the determined contrast potential based on the photosensitive member property re-estimated by the re-estimating section and thereby changes the image forming condition; and a determining section which repeats operations of the intermediate-density pattern forming section, the judging section, the laser power changing section, the re-estimating section, and the image forming condition changing section to obtain the agreement of the density of the intermediate-density pattern with the intermediate-density target value, and determines the laser power, the developing bias voltage, and the charging grid bias voltage.